The default palette of Highcharts is designed with accessibility in mind, so that any two neighbor colors are tested for different types of color blindness. In addition to that, there are a few ways to increase contrast, both for the visually impaired or for grayscale prints, but also for the charts to be more readable in general.
- Consider using monochrome color palettes.
- Consider using a high contrast theme.
- Consider applying dash styles to line series. This will make lines distinguishable even on poor black/white prints. See the live demo of dash styles.
- Consider applying a pattern fill to areas, columns or plot bands. This can be accomplished through the pattern fill module. For more information, see our documentation on pattern fills.
Keep in mind that pattern fills and dash styles could make your charts visually confusing and less accessible to some users, and that not all charts will be improved by adding these features. Subtle patterns are often preferred.